a) Field of the invention
The present invention relates to an objective optical system for endoscopes, and more specifically to an objective optical system for endoscopes which is configured to allow a plurality of adapter optical systems to be attached and detached thereto and therefrom for changing a direction toward a visual field, a field angle and an observation distance.
b) Description of the prior art
Conventionally, medical endoscopes which permit inserting elongated insert sections into living bodies for observing organs in the living bodies and passing forceps through forceps channels for sampling tissues of living bodies for detailedly diagnosing, diseased parts in detail are widely used. Industrial endoscopes which permit observation and inspections of interiors of boilers, turbines, chemical plants and so on are widely known in industrial fields.
Attached to an industrial endoscope, in particular, is a direct view type adapter for observing a diseased part which is located in the longitudinal direction before the insert section or a side-view type adapter for observing an inside wall which is located sideways in a direction perpendicular to the inserting direction. In practice, one simultaneously selects an adapter which has a field angle that is optimum for a location to be observed and another adapter which has an optimum observation distance (or a depth of field) and so on.
It is effective from an economical viewpoint to configure such an expensive endoscope as a tip adapter type endoscope which can be equipped with adapters having a direction of a visual field, a field angle and an observation distance matched with a location to be observed.
Also widely used in the industrial fields are electronic endoscopes which provide images of qualities remarkably improved owing to progress made in solid-state image pickup devices (CCD's).
A conventionally known example of such electronic endoscopes is an electronic endoscope disclosed by Japanese Patent Kokai Publication No. Hei 2-74,912 which has a composition shown in FIG. 1. This conventional electronic endoscope has no mechanism of the tip adapter type described above and is not versatile in the industrial fields. When an attempt is made to configure this electronic endoscope as the tip adapter type by dividing a lens system thereof into a subsystem which is located before an aperture stop S and replaceable with an adapter, and another subsystem located after the aperture stop S, for example, it is necessary to dispose a light guide to be used in an illumination system at a location of a lens unit L. A reason to select this disposition is that it is optimum to dispose a light guide G so that it turns below a side-viewing prism P as shown in FIG. 2 for attaching a side-viewing adapter.
When the light guide is disposed at the location of the lens unit L in the electronic endoscope disclosed by Japanese Patent Kokai Publication No. Hei 2-74,912, an objective optical system of this electronic endoscope has a large outside diameter at its tip. For disposing the light guide without enlarging its outside diameter, it is necessary to reduce an outside diameter of the lens unit L or a number of optical fibers which are to be used for composing a light guide. As a result, rays are eclipsed by the lens unit L or illuminating rays are reduced, thereby making brightness insufficient.
On the other hand, there is known an objective optical system for tip adapter type electronic endoscopes which has a composition shown in FIG. 3. This objective optical system for tip adapter type endoscopes is configured to concentrate all light guides at the location of the lens unit L for correcting the defect of the electronic endoscope disclosed by Japanese Patent Kokai publication No. Hei 2-74,912.
In the recent years where images of having higher qualities and full-screen sizes are strongly demanded, it is expected that electronic endoscopes which can provide images suited for display on high definition televisions (HDTV's) will be adopted in the near future.
For obtaining images of such high qualities, however, it is necessary to configure picture elements so as to have a smaller size, or lower illuminance per picture element on an image surface, thereby making it difficult to maintain the conventional image brightness. Further, it is known that depths of field are reduced by reducing sizes of picture elements.
For correcting the defects described above, it is necessary to reserve a required depth of field by enlarging an F number of an objective lens system and compensate for brightness by increasing a number of optical fibers that are used for making the light guide.
In the field of the endoscopes which should desirably have smaller diameters, however, it is undesirable to increase the number of optical fibers that are used for making the light guide since such increase results in enlarging the diameters of the endoscopes. In the case of the objective optical system for the tip adapter type of electronic endoscopes illustrated in FIG. 3, the objective optical system has an outside diameter which is enlarged by increasing the number of optical fibers. When the objective optical system for tip adapter type endoscopes is configured to provide a full-size screen, it may not accept an increase in the heights of rays caused by enlarging an image side and allow a visual field to be eclipsed. In FIG. 3, the reference symbol AD represents an adapter lens system and the reference symbol M designates a master lens system.